The present invention relates generally to the structure of a radio transceiver. Particularly, the invention relates to the structure of a high frequency part and intermediate frequency part of a radio transceiver, and to the selection of frequencies according to which it is possible to realize a device working in two frequency bands in an economical way.
In addition to the present cellular radio systems, new systems are being built in which the frequency band usually differs from that used in the old system and which, at least partly, operate in the same region with the old systems. Further, cellular radio systems operating in different frequency bands are already used in various parts of the world. From the user""s point of view, it would be advantageous if one single terminal (such as a mobile phone) could operate in several systems of different frequency bands in accordance with the user""s selection or a command given by the network. Important combinations of two systems and frequency bands are GSM and DCS1800 (Global System for Mobile telecommunications; Digital Communications System at 1800 megahertz). One form of DCS1800 is also called PCN (Personal Communications Network); in the present patent application, DCS1800 is called DCS for short. The transmission frequency band for a GSM terminal is 890-915 MHz, and the reception frequency band 935-960 MHz. The respective transmission frequency band for DCS is 1710-1785 MHz, and the reception frequency band is 1805-1880 Mz. For GSM, the band width or the extent of the transmission and reception frequency bands is 25 MHz, and 75 MHz for DCS; the duplex interval is respectively 45 MHz for GSM and 95 MHz for DCS. The simplest way to implement a radio transceiver with two frequency bands is to redouble all radio and intermediate frequency parts between the antenna and the baseband interface. However, this solution is relatively expensive and space consuming. A natural direction to find a more economical solution is to try to use as many common intermediate frequency parts as possible in both systems.
The Finnish patent application No. 941862 (Nokia Mobile Phones Ltd.) and the corresponding EP patent application published with the number EP-0 678 974, disclose a radio transceiver according to FIG. 1 operating in two frequency bands. Here the reception frequency signal FRX1 or FRX2 is first mixed to a first intermediate frequency IF1 in a mixer 4 by using the first mixing frequency LO1, and then to a second intermediate frequency IF2 in a mixer 6 by using a second mixing frequency LO2. Additionally, a third mixing frequency LO3 is used in demodulator 12 to demodulate the signal. The first and second intermediate frequencies remain unchanged irrespective of the frequency band one operates in. A different first mixing frequency is generated in accordance with the frequency band the radio transceiver operates in. For the generation of different first mixing frequencies, the apparatus has two separate UHF (Ultra High Frequency) frequency synthesizers 13 and 30. The abbreviation VCO in the figure stands for Voltage Controlled Oscillator. A common phase locked loop (PLL) 15 is used for adjusting the VCO frequencies. From the first intermediate frequency mixer 4 onwards, the same parts may be used in the receiver and, for example, it is not necessary to alter the second mixing frequency LO2 when transferring from one frequency band to another. A common modulator 21 is used for transmitting the signal, for which the necessary modulation frequency is obtained by dividing the second mixing frequency LO2 by an integer N in divider 33; further, a mixer 20 is used to which the said first mixing frequency LO1 is led as the mixing frequency. The radio frequency of a signal FTX1 or FTX2 to be transmitted is determined by which of the VCOs 13, 30 is used to generate the mixing frequency. Because of the two UHF-VCOs 13 and 30, this kind of apparatus requires relatively much current, its structure is large, it causes problems for the layout design, and it is expensive to manufacture.
The European published application No. EP 0 653 851 discloses a radio transceiver with two frequency bands in accordance with FIG. 2, containing only one UHF frequency synthesizer 31. The first mixing frequency LO1 generated by it is selected so that it is situated half-way between the two frequency bands in which the apparatus is supposed to operate. The publication presents an example in which the upper frequency band is 1710-1900 MHz, and the lower band is 890-960 MHz so that the first mixing frequency LO1 varies between 1290 and 1500 MHz, and the first intermediate frequency IF1 is 400 MHz. Consequently, when mixing onto the first intermediate frequency, lower LO injection is used for the first (higher) frequency band, and upper LO injection is used for the second (lower) frequency band. However, the problems related to this solution include the relatively wide tuning range required for the frequency synthesizer (at least 15% of the medium frequency of the frequency synthesizer) and the frequency hop which comes into question in such Time Division Duplexing (TDD) where different frequencies are used for reception and transmission. A specific feature of FIG. 2 is the use of the same mixer 30 both in transmission and reception. Switches 32 and 33 are used for separating reception and transmission from each other, and the switch 32 is additionally used for selecting the frequency band. Block 40 comprises intermediate frequency parts including e.g. the second intermediate mixer, and block 41 comprises the usual demodulation, decoding and D/A conversion parts. Block 43 comprises the A/D conversion needed for the transmission, and coding, and block 42 comprises the modulation needed for the transmission.
The object of the present invention is to introduce a new structure for a radio transceiver with two frequency bands, which is small and economical to manufacture, and which is suitable for use in a case when time division multiplexing is used in at least one of the two frequency bands.
These objects are achieved by selecting the mixing and intermediate frequencies so that the first mixing frequency of the receiver may, in a radio system of the first frequency band, be derived from the corresponding frequency of the second frequency by simple calculation process.
The radio transceiver of the invention is intended to be used for the reception and transmission of radio signals in two frequency bands, and it comprises a first signal input for the reception of radio signals in the first frequency band, and a second signal input for the reception of radio signals in the second frequency band. The invention is characterized in that it comprises
a first mixer connected to the first signal input for mixing the radio signals of the first frequency band onto a predetermined first intermediate frequency;
a second mixer for mixing said first intermediate frequency onto a predetermined second intermediate frequency; and
a third mixer connected to the second signal input for mixing the radio signals of the second frequency band onto a predetermined frequency which is substantially the same as said second intermediate frequency.
In a radio transceiver apparatus of the present invention, there is provided one UHF frequency synthesizer for generating the first mixing frequency, the frequency of the synthesizer being adjustable in a known way within a certain tuning range. The limiting values for the tuning range are selected so that, by adjusting the first mixing frequency, the receiver may be tuned in a known way to all frequencies in use in a first radio system. A block carrying out a certain simple operation, such as the division or multiplying of the frequency, may be connected in series with the UHF frequency synthesizer. The operation is selected so that, for tuning the receiver to frequencies used in a second radio system, when the oscillation produced by the UHF frequency synthesizer is led through said block, the series connection of the UHF frequency synthesizer and the block is capable of generating all the frequencies required by the second radio system without having to adjust the frequency of the UHF frequency synthesizer significantly outside said tuning range.
The received signal according to the radio system of higher frequency is still mixed a second time onto a certain lower frequency before the signal paths of the different systems are combined. When the radio transceiver is receiving a signal according to the radio system of higher frequency, it carries out three downconversions between reception and demodulation. A signal according to the radio system of lower frequency is only mixed down twice between reception and demodulation. The signal frequency of the radio system of higher frequency is after the second downconversion the same as the signal frequency of the radio system of lower frequency after the first downconversion.